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| GeoCommunity Mailing List |
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| Mailing List Archives |
| Subject: | RE: MI-L Albers Conical Equal Area |
| Date: |
04/06/2004 04:45:01 PM |
| From: |
Spencer Simpson |
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Josh C. Hickman wrote:
> Hello listers,
> I'm having a bit of a GIS brain fart... When a Map Projection for
> Pennsylvania says Albers Conical Equal Area, which projection does
> that relate to in MapInfo? I chose the Regional Equal-Area=20
> Projections from the drop down list and then selected the Equal-Area
> Projection (United States). Is this the same as Albers Conical Equal
> Area?? The problem I am experiencing is that when I load DOQQ data
> in that is in UTM 17, the vector data dosn't stretch to match the =
raster.
> This is a good indicator that the projection system I chose is wrong. =
=20
Josh:
The short answer is, you're going to have to go to whoever supplied your
vector data
(or dig through its documentation) and get all of the data's projection
information.
"Albers Conic" is simply not enough information.
The long answer is, terminology is a little confused here, since people
frequently use
the word "projection" to mean "coordinate system". =20
When you select a "projection" from a dialog box in MapInfo, you're =
really
selecting from a list of coordinate systems, despite the fact that many =
of=20
the coordinate systems call themselves "projections".
A projection is an abstract set of mathematical formulae for =
transforming
latitudes and longitude into a coordinate system. =20
A coordinate system associates coordinate pairs (i.e. (X, Y) values) =
with
points in some space. In MapInfo, this
space is usually the surface of the Earth.
When you put vector data in a map, and the data has a different =
coordinate
system from the one associated with the map,
MapInfo is able to use the definitions of the two coordinate systems to =
draw
the vector data in map's coordinate system.
To make a specific coordinate system from a particular projection, you =
need
to know which plug the values of several parameters=20
into the formulae (besides latitudes and longitudes). In the case of the
Albers Conic projection, you need:
- Datum
- coordinate units
- Latitude and longitude of origin
- the latitudes of two standard parallels
- a "false easting" and a "false northing"=20
MapInfo's "Equal-Area Projection (United States)" is really a coordinate
system.
I suspect it uses the Albers Conic Projection, since that's the best
equal-area=20
projection to use for a map of the entire United States. But the =
parameters
used=20
to make this coordinate system are probably not the same as the ones for =
the
coordinate system your vector data came from. So, you'll have to find =
out
those
parameter values.
Appendix H of the MapInfo User's Guide explains all of these concepts.
Hope this helps
Spencer
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